This invention pertains to a ground conductor monitoring system and method and, in particular, to a monitoring system for monitoring the ground conductor continuity in a power transmission system.
In electrical power transmission systems, a physical earth ground conductor suitably grounded at the power source, as by means of a grid buried in the earth, for example, is run along with the power cables to a load. In many applications of power transmission systems, such as a system extending into a mine for providing power to the various machines in the mine, it is quite important to insure at all times the continuity of the ground conductor so as to prevent an electrical shock hazard to persons in the mine operating the various machines. There is therefore a need for a system and method for insuring the continuity of such a ground conductor. Some techniques already exist in the prior art for monitoring ground conductor continuity. In accordance with such prior art techniques, DC signals have been coupled over such a ground conductor from the load to the source with monitoring means of an appropriate kind provided adjacent the power source and responsive to the absence of the DC voltage on the ground conductor (indicative of a break in its electrical continuity, for example) to actuate an alarm or trip a breaker. Typically, such DC signals have been on the order of 90 volts with currents as much as 5 amperes.
In addition to the obvious disadvantage of these DC systems requiring large amounts of power and placing a relatively high voltage on the ground conductor, they have also been subject to the disadvantage of not providing completely reliable monitoring. That is, in some instances there could be a break or discontinuity in the ground conductor which the monitoring system did not recognize as such. This is due to the fact that the earth has a relatively low resistance to DC signals, which can be on the order of a few ohms. This is particularly the case for portions of the earth which may contain certain kinds of mineral deposits. Thus, there might be a break in the ground conductor with an alternate path for the DC signal being established from the load through the earth to the earth grid and then to the receiver monitoring unit. Some AC systems are also subject to the possibility of alternate paths being established through the earth.
Other systems including some AC systems have been proposed which involve connecting equipment or circuit devices such as relays in series with the ground conductor being monitored. A serious disadvantage of such a system is that connecting these devices in series in the ground line can potentially destroy the integrity of the ground line. Thus if an electronic component, such as a diode or relay coil, which is connected in series with the ground line malfunctions, the integrity of the ground line itself can be destroyed. Thus an unsatisfactory situation is presented where devices intended to monitor the integrity of a ground line are themselves potentially capable of destroying the ground line's integrity.
Additionally, various of the prior art systems have also been subject to a phenomena known as false tripping. Where a monitoring current is provided in a ground line, with a receiver or relay or the like sensing the monitoring current, oftentimes a shunt parallel ground circuit from the machine or load back to ground at the power source will drain off sufficient current so that the relay or receiver generator an open circuit ground conductor alarm or actuates a circuit breaker. This occurs despite the fact that there is no problem in the ground conductor. This can occur for example when a second mining machine's frame is touching the frame of the one being monitored, thereby creating a parallel ground path back to ground at the load. Some prior art arrangements have been concerned with this problem of false tripping, but have attempted to solve it by connecting various additional devices in series with the ground conductor, thereby creating other problems.